For electronics, microelectronics and micro-electromechanics, semiconductor wafers with extreme requirements for global and local planarity, one-side referenced local planarity (nanotopology), roughness and cleanness are needed as starting materials (substrates). Semiconductor wafers are wafers of semiconductor materials, in particular compound semiconductors such as gallium arsenide and predominantly elementary semiconductors such as silicon and sometimes germanium.
According to the prior art, semiconductor wafers are produced in a multiplicity of successive process steps: in a first step, for example, a single crystal (rod) of semiconductor material is pulled by the Czochralski method or a polycrystalline block of semiconductor material is cast, and in a further step the resulting circular-cylindrical or block-shaped workpiece of semiconductor material (ingot) is cut into individual semiconductor wafers by wire sawing.
Wire saws are used in order to cut a multiplicity of wafers from a workpiece made of semiconductor material. DE 195 17 107 C2 and U.S. Pat. No. 5,771,876 describe the functional principle of a wire saw which is suitable for the production of semiconductor wafers. The essential components of these wire saws include a machine frame, a forward feed device and a sawing tool, which consists of a web (wire web) of parallel wire sections. The spacing of the wires in the wire web depends on the desired target thickness of the wafers to be cut, and for semiconductor material wafers is for example from 100 to 1000 μm.
DE 101 47 634 B4 describes a method for cutting wafers of semiconductor material from a single crystal, the single crystal being rotated about a longitudinal axis during the cutting of the semiconductor wafers while penetrating into the sawing wires of two wire webs of a wire saw, the sawn wafers of semiconductor material having parallel rotationally symmetrical curved sides.
In general, the wire web is formed by a multiplicity of parallel wire sections which are tensioned between at least two wire guide rollers, the wire guide rollers being rotatably mounted and at least one of them being a driven roll.
The wire sections may belong to a single finite wire, which is guided spirally around the system of rollers and is unwound from a stock spool onto a receiver spool. Patent specification U.S. Pat. No. 4,655,191, on the other hand, discloses a wire saw in which a multiplicity of finite wires are provided and each wire section of the wire web is assigned to one of these wires. EP 522 542 A1 also discloses a wire saw in which a multiplicity of endless wire loops run around the system of wire-guide rollers.
During the cutting process, the workpiece passes through the wire web, in which the sawing wire is arranged in the form of wire sections lying parallel to one another. The passage through the wire web is brought about by means of a forward feed device, which moves the workpiece against the wire web, the wire web against the workpiece or the workpiece and the wire web against one another.
When the wire web penetrates into the workpiece, according to the prior art, for a defined time and with a particular speed, a defined length of the sawing wire is fed forward (wire forward) and a further defined length is fed back (wire backward), the backward length WBL generally being shorter than the forward length (WFL). This sawing method is also referred to as a reciprocating movement method and is disclosed, for example, in DE 39 40 691 A1 and in US 2010 1630 10 A2.
EP 1 717 001 B1 teaches that a forward movement and a backward movement are carried out when sawing a workpiece with a wire saw, the length of the wire during the backward movement (WBL) being shorter than the length of the wire during the forward movement (WFL).
DE 11 2008 003 321 T5 discloses, for removal of the wire web from a sawn workpiece, a wire running length in the forward and backward directions respectively of 1 m or less and a wire running speed of 2 m/min or less.
The sawing of a workpiece into many wafers with a wire saw is carried out in the presence of a liquid cutting medium, which inter alia is used to transport the material abraded by the sawing wire out of the sawing kerf, and according to the prior art is applied onto the sawing wire.
If the sawing wire is covered with an abrasive coating, for example diamond, a cutting medium without free abrasive is generally used. When using wire saws comprising a sawing wire without fixedly bonded abrasive, the abrasive is supplied in the form of a suspension (cutting medium suspension, sawing slurry, slurry) during the cutting process.
When cutting wafers from a workpiece made of semiconductor material, it is conventional for the workpiece to be connected to a sawing strip into which the sawing wire cuts at the end of the process. The sawing strip is for example a graphite strip, which is adhesively bonded or cemented on the lateral face of the workpiece. The workpiece with the sawing strip is then cemented on a support body. After the cutting, the resulting semiconductor wafers remain fixed on the sawing strip like the teeth of a comb, and can thus be removed from the wire saw. Subsequently, the remaining sawing strip is separated from the semiconductor wafers.
The production of semiconductor wafers from workpieces made of semiconductor material, for example from a circular-cylindrical rod of a single crystal or a cuboid polycrystalline block, places great demands on the wire sawing. The aim of the sawing process is generally for each sawn semiconductor wafer to have side faces which are as planar as possible and lie parallel to one another.
Deviations from the ideal wafer shape are described, inter alia, by the parameters warp and bow.
The so-called warp of the wafers is a known measure of the deviation of the actual wafer shape from the desired ideal shape. The warp should generally amount to at most a few micrometers (μm).
The bow is a measure of the convex or concave deformation of a wafer and should in general be at most a few micrometers (μm).
The warp and bow of the wafers are essentially caused by a relative movement of the sawing wire sections relative to the workpiece, which takes place in the axial direction with respect to the workpiece in the course of the sawing process. This relative movement may for example be the result of cutting forces which occur during the sawing, axial displacements of the wire guide rollers due to thermal expansion, bearing plays or thermal expansion of the workpiece.
US application US 2010/0089377A1 teaches a method for cutting a multiplicity of wafers from a workpiece, in which the warp and the bow of the wafers are reduced. To this end, the displacements of the workpiece and of the wire web in the axial direction are respectively measured and correspondingly adapted.
During the cutting of wafers from a workpiece, strong mechanical and thermal loading of the sawing wire in the wire web occurs, which can lead to unplanned interruption of the wire sawing process due to wire breaking (wire fracture).
In order to avoid wire breakages when cutting a workpiece into a multiplicity of wafers by means of a wire web, laid-open specification DE 10 2011 008 397 A1 teaches the application of a torque detection device on the stationary axle of a deflection pulley, so that an excessively large tensile stress applied to the sawing wire can be avoided.
In the event of a wire breakage, the sawing process must be interrupted as rapidly as possible in order to avoid damage to the wire saw and the material to be cut.
In order to be able to identify a wire breakage immediately and to be able to stop the wire saw process within the shortest possible time, WO 2011/151022 A1 discloses a method for monitoring wire breakages when cutting a workpiece by means of wire webs, in which a direct current is passed through the wire web and generates a voltage, which is monitored by a sensor, over the wire array. In the event of a voltage deviation caused by a wire breakage, the cutting process is interrupted automatically.
After a wire breakage and switching off of the wire saw, the workpiece and the wire web are separated from one another. To this end, for example, the workpiece may be removed upward from the web. After repair, the workpiece is reintroduced into the wire web, optionally with minor movement of the sawing wires in the presence of the cutting medium.
The interruption of the wire sawing process should be as brief as possible, since the sawing wires and guide rolls heated by the cutting into the workpiece cool during shutdown and contraction of the sawing wires and guide rolls may occur. This may lead to impaired nanotopography of the wafer surfaces on resumption of the sawing process.
In order to avoid contraction of the heated sawing wires and guide rolls on unplanned shutdown, publication DE 11 2009 001 747 T5 discloses a method for resuming operation of a wire saw, in which, during the unplanned shutdown, thermal regulation of the workpiece and of the relevant parts of the wire saw (sawing wire and guide rolls) is ensured by the liquid cutting medium. In addition, a displacement magnitude for the guide rolls of the wire web, which corresponds to the displacement of the workpiece on interruption of the process, is set in the axial direction.
For resuming the sawing process, however, the teaching of publication DE 11 2009 001 747 T5 does not take into account the fact that the sawing wire has at least partially a different degree of wear after repair than before the interruption. A different degree of wire wear results in a different diameter of the wire.
For this reason, a deep incision (groove, indentation) may occur in the surface of the wafers to be cut when resuming the wire sawing process, so that in the least favorable case the wafers are no longer suitable for further processing owing to the inferior surface topography. It is this problem which gives rise to the object of the present invention.